Because I'm always struggling, a note for myself (and for others) how it works.
Installing PHP PECL IMagick on Ubuntu:
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1. $ sudo aptitude install php-pear imagemagick php5-dev libmagick9-dev
2. $ sudo pecl install Imagick
3. $ sudo nano /etc/php5/apache2/php.ini
4. enter extension=imagick.so -> save (ctrl+x / Y)
5. $ sudo apache2ctl configtest
6. $ sudo apache2ctl stop
7. $ sudo apache2ctl start